The installation of the equipment for the microgrid system was completed during previous NPS thesis research projects [16], [18], [22]. The components, however, had not been integrated into a working system. In order to implement the desired control strategy, communications had to be established between a centralized controller and the equipment. The initial focus of this research was therefore setting up a communications network to be used by the centralized controller. Figure 16 illustrates this communications network.
Figure 16. Communications Network
The central microgrid controller uses a USB serial port adapter to communicate with the heater system, Modbus communications protocol over Ethernet to communicate with the microgrid inverters, and BACnet communications protocol over Ethernet to communicate with the chiller system. However, since BACnet commands are disabled, an analog output device is used to vary the chiller power. MATLAB was used to develop functions that could execute each of these communications protocols. A separate function was developed for each protocol.
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1. SMA Sunny WebBox Webpage
The SMA Sunny WebBox collects information from the inverters and uses that data to generate a webpage. Information on the webpage updates every 30 seconds. The WebBox connects to the computer via Ethernet. Figure 17 shows an example of data collected from the master Sunny Island inverter that is displayed by the webpage.
Figure 17. Example Sunny Island Data on Webpage
Data is read from the web page using a URL filter MATLAB script that scans the page for given variables. The URL filter function has numerous drawbacks. It takes over two minutes to scan the necessary data, sometimes the filter scans the wrong values, and if the user navigates the webpage while the function is scanning it will cause an error. The webpage also has to be logged in for the function to work. The function was required to scan the page a minimum of every five minutes to remain logged in to the website. The URL Filer function was eventually replaced with the Modbus function as it avoids many of these issues.
2. Modbus Protocol
The SMA microgrid uses Modbus communications protocols. The following information from the SMA Sunny WebBox Modbus Interface describes the variation of the Modbus communications protocol for SMA products.
Modbus Protocol
The Modbus Application Protocol is an industrial communication protocol that is currently mainly used in the solar sector for plant communication in PV power stations.
The Modbus protocol has been developed for reading data from or writing data to clearly defined data areas. The Modbus specification does not specify what data is within which data area. This information must be defined specifically for a device in a so-called Modbus profile. With knowledge of the specific Modbus profile, a Modbus master (e.g., a SCADA system) can access the data of a Modbus slave (e.g. Sunny WebBox).
The SMA Modbus profile is the special Modbus profile for SMA devices.
SMA Modbus Profile
The SMA Modbus profile contains definitions for selected SMA devices. For the definition there was a reduction of the available data and an assignment to the respective Modbus registers. The SMA Modbus profile contains for example overall and daily energy, current output, voltages and currents. The assignment between SMA device data and Modbus addresses is divided into sections in the SMA Modbus profile that can be addressed by Unit IDs.
In order to enable access to data of an SMA device, a special gateway is required that is provided by Sunny WebBox.
Plant Topology
The SMA Modbus profile has been designed for a hierarchical plant structure. This structure contains the WebBox as communication device that is equipped with a Modbus TCP/IP interface. All other SMA devices that are connected to the WebBox via the SMA fieldbus are subordinate to it.
From the perspective of the Modbus protocol, the WebBox is a Modbus slave that provides a gateway to SMA devices. The SMA devices can only be addressed using this gateway per Unit ID. [23]
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Modbus communications was incorporated into the control scrip in order to improve the communications between the controller and the microgrid. It allows the controller to bypass the webpage generated by the Sunny WebBox and request data directly from the Sunny Boy and Sunny Island inverters. This dramatically speeds up communications, allowing access to data as it becomes available. For example, the webpage displays data for all of the inverters, updating approximately every 30 seconds. When polling the inverters directly, however, some values update as frequently as every 5 seconds.
Using Modbus communications, the controller polls data from the three Sunny Boy inverters and the master Sunny Island inverter. Each inverter has a specified Unit Identifier (ID). Within each inverter, data is stored in specified register addresses. Most values are stored in two adjacent registers as unsigned 32-bit integers (UNIT32). The controller converts these values to decimal values. The query and response of all the data takes less than two seconds. The controller only polls the inverters every 20 seconds, allowing time for all the values to update. Incorporating Modbus communications has drastically improved the performance of the controller.
3. Serial Port
The Steffes heater uses a universal serial bus (USB) serial port adapter for communications. The MATLAB control scrip communicated with the heater using a USB connection. Using a communications (COM) port, requests could be sent to the heater to adjust the element on percent, fan setting, and read the temperature as well as the element on percent and fan setting.
A serial port is a general-purpose serial communications interface. Serial ports use the RS-232 telecommunications standard for data transmission. Most modern computers no longer have serial ports and require a serial-to-USB converter to interface with RS- 232 serial devices [24]. A virtual COM port (VCP) driver from FTDI Chip was used to convert the USB device to an additional COM port. The USB device could then be accessed as a standard COM port using application software such as MATLAB [25].
4. BACnet Protocol
The Trane chiller system uses BACnet communications protocol. The Building Automation and Control Networks (BACnet) is an American national standard data communications protocol developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) [26]. The following information on BACnet protocol is from the Trane Integration Guide for BACnet Communication Interfaces.
The Building Automation and Control Network (BACnet and ANSI/ASHRAE Standard 135–2004) protocol is a standard that allows building automation systems or components from different manufacturers to share information and control functions. BACnet provides building owners the capability to connect various types of building control systems or subsystems together for a variety of reasons. In addition, multiple vendors can use this protocol to share information for monitoring and supervisory control between systems and devices in a multi-vendor interconnected system.
The BACnet protocol identifies standard objects (data points) called BACnet objects. Each object has a defined list of properties that provide information about that object. BACnet also defines a number of standard application services that are used to access data and manipulate these objects and provides a client/server communication between devices. [27]
The Trane chiller system uses a Tracer UC600 Programmable controller along with two unit controllers, a UC600 and a UC400. The following information on these controllers is from the Trane website.
The Tracer® SC is an intelligent field panel that communicates with unit controllers (BACnet) that provide standalone control of HVAC equipment. The Tracer® SC scans all unit controllers to update information and coordinate building control, including building subsystems such as VAV and chilled water systems. The LAN allows building operators to manage these varied components as one system using web access. [28]
Trane offers programmable BACnet controllers that can be customized to specific needs. The Tracer UC400 and Tracer UC600, which are also used as airside controllers, are compatible with a wide variety of application scenarios, and support graphical programming and the BACnet protocol. These two programmable BACnet controllers are designed to work with Tracer SC and third-party BACnet MS/TP systems. [29]
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The Tracer SC controller is similar to the SMA Sunny Webbox in that it receives and stores data from the unit controllers. The Tracer SC controller uses this data to generate a webpage and to update the Trane control panel. It monitors data from the entire system, including compressor and pump speeds, glycol-water temperatures, and ice storage levels The Tracer SC controller is also intended to implement controls, but that feature was not used for this project.
BACnet communications was integrated into the controller to allow the controller to poll data from the Tracer SC controller. The controller can also communicate directly with the two unit controllers. The UC400 controller controls the compressor and pump speeds. The controller can send BACnet commands to the UC400 controller to control these speeds. However, turning off the control feature of the Tracer SC override this feature as well. Therefore, speeds commands are sent to the UC400 controller using an analog signal.
5. NATIONAL INSTRUMENTS COMPACT DATA ACQUISITION
A National Instruments (NI) compact data acquisition (cDAQ) device is a modular platform that can be used to interface with different sensors and signals. The NI cDAQ is used with the NI 9263 analog output module to send analog signals to the chiller. These signal are received by the UC400 controller and modulate compressor speed. This adjusts the power to the chiller. The NI cDAQ chassis and analog output module are shown in Figure 18. The module is used to output 0–10V, which corresponds to 0–100% compressor speed.
Figure 18. NI cDAQ Analog Output Device